The invention relates to a method for the self-diagnosis of an exhaust gas probe, the exhaust gas probe having at least one heating element.
Furthermore, the invention relates to an apparatus for the self-diagnosis of a corresponding exhaust gas probe, the exhaust gas probe being connected to an engine controller or a sensor control unit, and the engine controller or the sensor control unit having devices for the diagnosis of the exhaust gas probe.
Particle sensors are used nowadays, for example, for monitoring the soot emission of internal combustion engines and for on board diagnosis (OBD), for example functionally monitoring particle filters. Here, collecting, resistive particle sensors (particle mass sensors or PM sensors) are known which evaluate a change in the electric properties of an interdigital electrode structure on account of particle accretions. Two or more electrodes can be provided which preferably engage into one another in the manner of a comb. The electrodes are short circuited as a result of a rising number of particles accreting on the particle sensor, which results in an electric resistance which decreases as the particle accretion increases, a decreasing impedance or in a change in a characteristic variable which is linked to the resistance or the impedance, such as a voltage and/or a current. For evaluation, generally a threshold value, for example of a measuring current between the electrodes, is fixed and the time until the threshold value is reached is used as a measure of the accreted particle quantity. As an alternative, a signal change speed during the particle accretion can also be evaluated. If the particle sensor is fully loaded, the accreted particles are burned in a regeneration phase with the aid of a heating element which is integrated into the particle sensor.
DE 101 33 384 A1 describes a resistive particle sensor of this type. The particle sensor is constructed from two comblike electrodes which engage into one another and are covered at least partially by a trapping sleeve. If particles from a gas stream accrete on the particle sensor, this leads to a change in the impedance of the particle sensor, which change can be evaluated and from which the amount of accreted particles and therefore the amount of particles entrained in the exhaust gas can be deduced.
DE 10 2005 015103 A1 describes one special embodiment of a particle sensor of this type, in which embodiment the actual soot sensor is surrounded with a metallic housing for protection. Inter alia, one embodiment is described, in which the soot sensor is arranged in a protective tube which surrounds it or, as an alternative, a double protective tube which has one or more inlet openings for the gas stream.
In order to regenerate the particle sensor after particle accretion has taken place, the sensor element has to be burned free with the aid of an integrated heating element. This has to be carried out at defined time intervals, in order to avoid corruptions of the particle concentration determination.
For the self-diagnosis, there is provision, for example, in a further application of the applicant with the internal reference R.318399 for the particle sensor to have an additional flat test electrode, and for different test voltages to be applied between the measuring electrodes and the test electrode in a plurality of method steps, and for in each case a current or a capacitance to be measured and for correct functioning of the particle sensor to be deduced using the determined values.
EP 1 925 926 A1 describes an apparatus and an evaluation method for testing the functionality and for carrying out a plausibility check of a sensor which is based on an electrode system, in particular of a particle sensor, the apparatus comprising at least one reference electrode system. Here, the evaluation method provides comparative measurements at the measuring electrodes and at the reference electrodes, correct functioning of the sensor being deduced using the determined values.
Since the particle sensor is arranged downstream in the exhaust gas stream with regard to a particle filter if used for on board diagnosis, no more particles, in particular soot particles, which could supply a corresponding sensor signal, should be situated in the exhaust gas in the case of a fully functional particle filter at that point, at which the particle sensor is arranged. However, the fact that no signal is supplied by a sensor can also mean that the particle sensor is defective and therefore a possibly likewise defective particle filter is not detected as being defective. In addition, sooting of the protective tube can also lead to a faulty interpretation of the signals.